One important aspect of ensuring reliable and safe operation of nuclear power plants includes reprocessing liquid containing radioactive material resulting from their service. Evaporation is a method normally employed for reprocessing radiocontaminated liquids. During evaporation the liquid is subjected to evaporation whereby concentration of radioactive contaminants contained in the liquid is considerably increased. Therefore, as a result of evaporation the liquid containing radioactive material is greatly reduced in volume which simplifies its subsequent treatment.
The use of the evaporation process posses two problems, particularly the organization of biological protection to ensure the safety of attending personnel and obtaining a high quality vapor for further utilization in the nuclear power plants.
Normally, biological shielding is installed around evaporators intended for reprocessing liquid containing radioactive material and generally has the form of concrete walls and encasements, whereas high quality of vapour is attained by using in the evaporators means for vapor washing. Therewith, the higher is the concentration of radioactive contaminants in the liquid, the higher is the radiation level at the surface of the evaporator and the lower is the quality of vapor obtained. As a consequence, the biological shielding must be thicker and the means for vapor washing more elaborate.
There is known a method of continuous evaporation of liquid containing radioactive material by evaporation residing in passing the liquid containing radioactive material through one evaporation zone, discharging the liquid containing highly concentrated radioactive contaminants and washing the vapor generated in the course of the process (cf. L. S. Sterman, S. A. Tevlin and A. T. Sharkov "Teplovye i atomnye elektrostantsii"--Steam and Nuclear Power Plants, in Russian, the Energoizdat Publishers, 1982, p.166, FIG. 9.30). This method, however, suffers from a disadvantage in that the biological shielding must enclose the entire evaporation zone during the treatment process, which entails the use of a very bulky biological protection assembly.
There is known another method of continuous evaporation of radiocontaminated liquids by evaporation comprising successive passing of a continuously fed radiocontaminated liquid through two evaporation zones isolated from the outside by a layer or body of liquid to prevent radioactive radiation for successively increasing the concentration of radioactive contaminants present in the liquid, discharging the liquid containing the concentrated radioactive contaminants from the second evaporation zone, washing the vapor produced in each of the two evaporation zones to remove the radioactive contaminants present in the vapor, and discharging the thus washed vapor (cf. above L. S. Sterman en al . . . p. 167, FIG. 9.31b).
Compared with the previously described prior art method, the lastdescribed one is advantageous in that the highest concentration of radioactive contaminants is attained in the second evaporation zone, rather than in the entire volume or flow of the liquid being treated. In view of this fact, the biological shielding of the first evaporation zone, wherein concentration of radioactive contaminants and consequently the level of radioactive radiation are lower than in the first evaporation zone, may be less bulky.
However, at a high concentration of radioactive contaminants in the liquid being processed, radioactive radiation from the second evaporation zone to the outside is increased, which hampers a further concentration of radioactive contaminants in the liquid without a more reliable biological shielding, which either affects the efficiency of the apparatus for continuous evaporation of liquid containing radioactive material or requires a more thorough and reliable biological shielding.
There is known an evaporator comprising a housing having pipes for feeding and discharging a heating medium, a pipe for discharging a vapor produced in the evaporator, and pipes for admitting and discharging a liquid being treated. The housing accommodates a tubular heating chamber and a means for vapor purification (cf. L. S. Sterman "Ispariteli"--Evaporators, in Russian, the Mashgiz Publishers, Moscow, 1956, pp. 16 and 17, FIG. 4).
The above evaporator is arranged horizontally, whereby the quality of vapor obtained is not sufficiently high due to the impossibility of employing effective means for vapor washing requiring considerable space inside the evaporator housing. In addition, the horizontal arrangement of the evaporator requires much production area.
These disadvantages are partially obviated in a system of continuous evaporation of liquid containing radioactive material comprised of two intercommunicable vertical evaporators (cf. L. S. Sterman "Ispariteli" . . . , pp. 22 to 24, FIG. 10).
This system does not need a strong biological shielding for the two evaporation zones, since the high degree of concentration of radioactive contaminants and consequently high level of radiation are associated exclusively with the second evaporation zone, that is the second evaporator.
However, biological shielding of the second evaporator must be reliable enough to ensure safe working conditions for the attending personnel when concentration of the radioactive contaminants in the liquid being treated is high.
One more apparatus (evaporator) for continuous evaporation of liquid containing radioactive material which bears the closest resemblance to the one to be described in the present specification is known to comprise a vertical housing having pipes for feeding a liquid containing radioactive material, pipes for feeding and discharging a heating medium and a pipe for discharging vapor generated in the evaporator. The apparatus accommodates a tubular heating chamber and an evaporation chamber containing some of the tubes of the tubular heating chamber, made open in the upper section for making way to the vapor escaping to the interior of the housing and provided with a means for feeding thereinto the liquid being treated and a pipe for discharging the liquid outside the housing; a means for vapor washing being provided above the heating and evaporation chambers (cf. British Pat. No. 1,347,354, IPC B 01 D 1/12, 1/26, published 1974).
The evaporation chamber is defined by radially extending partitions separating a section of the housing interior including some of the tubes of the heating chamber and some of the volume occupied by the liquid being treated. Upper ribs of the partitions are arranged to overlie the level of liquid being treated. In this manner two evaporation zones are provided in one evaporator.
The liquid to be treated is supplied to the housing where it is partially evaporated whereafter the liquid is fed through a port provided in the partition to the evaporation chamber for further evaporation.
As distinct from the heretofore described prior art apparatus, highly concentrated contaminants tend to collect in the evaporation chamber rather than in the entire interior of the apparatus.
In view of the foregoing arrangement, increased radioactive radiation through the housing of the apparatus to the outside will occur only in the portion thereof where the evaporation chamber is disposed.
Because a substantial increase in the concentration of the radioactive contaminants fails to occur throughout the entire volume of the evaporator, the average concentration of these contaminants in the liquid occupying the interior of the evaporator is considerably lower than in the previously described prior art apparatus. Taking account of the fact that radioactivity of vapor is determined by concentration of the radioactive contaminants in the droplets of liquid carried by the vapor from the liquid being treated, a higher quality of vapor is attained in this apparatus. However, due to the increased concentration of radioactive contaminants in the location where the evaporation chamber is disposed, a more formidable biological shielding is required, which in turn leads to more expences for the construction of the apparatus. In addition, another disadvantage resides in that the quality of vapor is still not sufficient because this vapor carries from the evaporation chamber to the interior of the housing droplets of liquid containing concentrated radioactive contaminants.